1. Field of the Invention
The present invention relates in general to a fiber-optic rotation sensor for measuring a rotation rate, and more particularly to a fiber-optic rotation sensor using a rare-earth doped fiber laser.
2. Description of the Prior Art
With reference to FIG. 1, there is schematically shown a construction of a prior art fiber-optic rotation sensor using an optical fiber. In the prior art fiber-optic rotation sensor, the light emitted from a light source 11 is received by a first directional coupler 51 where the incident light is divided into two parts which are to travel in individual fibers outputted from the directional coupler 51. Here, one of the fibers outputted from the first directional coupler 51 is connected to a polarizer 55 while the other fiber is connected to no element. The polarizer 55 is in turn connected to a second directional coupler 50 which is included in a fiber rotation sensing unit 82. The prior art fiber-optic rotation sensor further includes a phase modulator 54 which is placed in the rotation sensing unit 82 for causing a sensitive measurement of a rotation rate. The first directional coupler 51 is also connected to a photo detector 63 by which an optical signal or an interference signal outputted from the first directional coupler 51 is detected.
In operation of the above prior art fiber-optic rotation sensor, the light emitted from the light source 11 is received by the first directional coupler 51. Upon reception of the light, the first directional coupler 51 divides the incident light into two parts and outputs the two light parts through individual fibers outputted therefrom. One of the light parts outputted from the first directional coupler 51 is in turn received by the second directional coupler 50 of the fiber rotation sensing unit 82 through the polarizer 55 and divided into two light parts therein. In the rotation sensing unit 82, the two light parts from the second directional coupler 50 travel in the fiber coil of the unit 82 in opposed directions and return to the second directional coupler 50. When the two light parts travel in the fiber coil of the unit 82 in opposed directions prior to returning to the second directional coupler 50 as described above, there occurs an interference between the lights. The lights, generating the interference therebetween and returning to the second directional coupler 50, are in turn transmitted to the photo detector 63 through the polarizer 55 and the first directional coupler 51 in series. Upon reception of the lights, the photo detector 63 detects an optical signal or an interference signal from the lights.
At this time, in the case of no rotation of the fiber rotation sensing unit 82, there is reciprocal phase difference between the two lights travelling in the fiber coil of the unit 82 in opposed directions. However, when the fiber rotation sensing unit 82 is rotated, there is a nonreciprocal phase difference between the two lights travelling in the fiber coil of the unit 82 in opposed directions. Such a nonreciprocal phase difference is detected by the photo detector 63, thus to make it possible to measure the rotation rate.
However, it is noted to those skilled in the art that the prior art fiber-optic rotation sensor has a problem in that it should require a substantially complex signal processing in order to measure the rotation rate from the detect signal of the photo detector 63.